Multiple powder transfer in photoelectrostatic duplicator

ABSTRACT

A PROCESS FOR DUPLICATING ON PLAIN PAPER UTILIZING ELECTROSTATIC TECHNIQUES TO REPEATEDLY PRODUCE A TRANSFERABLE POWDER IMAGE ON A PHOTOELECTROSTATIC MEMBER COATED WITH A LAYER OF PHOTOCONDUCTIVE MATERIAL AND SUCCESSIVELY TRANSFERING THE POWDER IMAGE TO THE PLAIN PAPER. TRANSFER OF THE POWDER IMAGE FROM THE PHOTOCONDUCTIVE LAYER IS ACCOMPLISHED UNDER STRICTLY CONTROLLED CONDITIONS OF PRESSURE MAINTAINED IN THE RANGE OF 2 TO 8 POUNDS PER SQUARE INCH WHILE OPTIONALLY IMPOSING AN ELECTRICAL FIELD BETWEEN THE RECEIVING SHEET AND THE PHOTOELECTROSTATIC MEMBER IN THE RANGE OF 0-3000 VOLTS.

Dec. 14, E HELF- 0 3,627,523

MULTIPLE POWDER TRANSFER IN PHOTOELECTROSTATIC DUPLICATOR Filed March 14, 1968 4 Shoots-Shoot 1 HIGH O VOLTAGE T SOURCE HIGH mums: 6 0 SOURCE 6g 60 50k g g I Dec. 14, 1971 SHELFFQ 3,627,523

MULTIPLE POWDER TRANSFER IN PHOTOELECTROSTATIG DUPLICATOR Filed March 14, 1968 4 Sheets-Sheet 2 I n f 72 l I v I SOURCE ""j 4 1 0 100 men VOLTAGE 110 0 J02 l1 SOURCE [08 men v w/ace 1/ 1 9.9 SOURCE HIGH VOLTAGE SOURCE 72 1 1 f M i- \Q- (I SOURCE SOURCI: I 13 K l MMA,

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L. E. SHELFFO Dec. 14, 1971 MULTIPLE POWDER TRANSFER IN IUOTOELEC'I'ROS'I'A'IIC DUPLICATOR 4 Sheets-Sheet 5 Filed March 14, 1968 immmmzmmnnmp NWH United States Patent Office 3,627,523 Patented Dec. 14, 1971 3,627,523 MULTIPLE POWDER TRANSFER IN PHOTO- ELECTROSTATIC DUPLICATOR Loren E. Shellfo, Palatine, Ill., assignor to Addressograph- Multigraph Corporation, Mount Prospect, Ill. Filed Mar. 14, 1968, Ser. No. 713,111 Int. Cl. G03g 13/00, 13/14 US. Cl. 96-1.4 9 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates generally to photoelectrostatic reproduction and, more particularly, to improved methods of making multiple copies from an original employing electrostatic techniques.

Photoelectrostatic techniquesfor making single copies of a graphic original on a treated or coated copy paper are well known. These techniques are described in an article entitled Electrofax Direct Electrophotographic Printing on Paper, by C. J. Young and H. G. Greig, RCA Review, volume 15, No. 4, pp. 469-484, December 1954. The basic Electrofax process involves the steps of applying a blanket electrostatic charge on the coated side of the paper while in the dark, exposing the charged surface to a pattern of light and shadow in order to form a latent electrostatic image on the photoconductive surface of said paper, the image then being developed by applying an electrostatically attractable pigmented resin powder. This process has been eminently successful in producing copies directly on a flexible substrate, usually paper, which has been coated with photoconductive zinc oxide particles dispersed in film-forming insulating resin binder. In the circumstance when multiple reproductions of a graphic original were required the complete process, including all steps, were repeated for each reproduction.

Other known techniques for electrostatic copying are based on the use of elemental selenium as the photoconductive medium on which a latent electrostatic image is produced, developed with suitable electroscopic powder, the powder image transferred to an untreated sur face, such as plain paper, where it is then fixed by such known techniques as heat fusion or solvent vapor fixing. Subsequent use of the selenium medium requires that the old image be removed or erased. Here again, multiple reproductions are achieved by repeating the full cycle for each transferred reproduction.

The processes above described are considered photoelectrostatic copying processes as distinguished from duplicating techniques. For the purposes of this discussion, duplicating involves the production of multiple reproductions from a single image without repeating the full sequence of steps necessary to produce the initial image. For each reproduction the image on the photoconductive member is redusted with the electroscopic powder so that in effect it now becomes a master.

The advantages derived from the known electrostatic copying processes heretofore have not been successfully employed in duplicating. In the case of making reproductions on zinc oxide coated papers, the reproduction of a large number of editions from a single original hecomes costly and is slow when compared to such known duplicating techniques as conventional lithographic equipment, spirit duplicators and the like. Transfer techniques involving imaging a selenium coated cylinder and then transferring the powder image from the cylinder onto plain paper requires erasing and reimaging the cylinder after each copy.

It has been suggested that the reimaging step may be eliminated, that is, the latent electrostatic image may be reused merely by redusting with electroscopic powder and transferring the redusted image to a receiving sheet by the use of corona means. Such techniques recognize the problem of preserving the latent electrostatic image over repeated transfers and separations of the receiving sheet from the photoconductive member. Such systems require separation of the receiving sheet to take place under strictly controlled electrical field conditions at the point of separation. The voltage parameters creating the electrical field suggested 'by the heretofore known processes tend to limit the density of the copy while preserving the latent image or else requires the use of a corona discharge to accomplish more effective transfer of theelectroscopic powder to the receiving sheet and an additional control at the point of separation.

SUMMARY OF INVENTION In accordance with this invention it has been found that multiple copies of a graphic original may be duplicated on plain paper through the transfer of a material image formed on a photoelectrostatic member corre sponding to the graphic subject matter on the original.

A conventional photoelectrostatic member is employed comprising finely divided photoconductive Zinc oxide particles dispersed in an insulating resin binder applied to a suitable substrate, usually paper. The zinc oxide photoelectrostatic member is given a blanket electrostatic charge, exposed to a pattern of light and shadow creating thereon a latent electrostatic image. A latent image thus produced represents an image receiving medium on the photoconductive layer which is developed into a material image and then transferred under strictly controlled conditions to a receiving sheet. Under proper conditions of transfer the image receiving medium can survive the transfer step of the material image to the receiving sheet so that it may subsequently be redeveloped and the next image transferred, giving rise to a duplicating process capable of long runs.

The photoelectrostatic member with its image receiving medium is disposed on a rotatable cylinder so that the succeeding steps of the duplicating process can be arranged in a planetary fashion about the periphery of the cylinder path.

The latent image bearing member disposed on thecylinder is brought to the developing station where electroscopic particles are applied to the surface of the member by means of the well-known magnetic brush technique. It will be appreciated that other means for applying electroscopic powder may be employed such as powder cloud generator, cascading the developer over the latent image, or the use of liquid systems.

In the instant process it has been found advantageous to use the magnetic brush technique and to apply an adjustable DC potential to the brush so as to produce an electrical field between the brush and the latent electrostatic image. The brush biasing technique is advantageous in controlling the development of the image to the proper level of contrast.

A feed station is provided from which is fed the receiving sheet in synchronization With the rotation of the cyl- 3 inder so that the receiving sheet and the imaged portions to be transferred are in registration.

A transfer zone is established along the planetary path of the cylinder which provides the proper pressure and electrical field conditions to optimize image transfer. The transfer zone includes a transfer member which comprises a roller having a resilient outer layer. Optionally, the roller may be connected to a high DC voltage supply which forms one electrode of a field producing arrangement and the metal support carrying the photoelectrostatic member serves as the other electrode connected to ground. The outer layer of the transfer member is of resilient material such as rubber and the axis of rotation of the roller is adjustable with respect to its distance from the surface of the cylinder so that the transfer member may be positioned to provide varying pressures on the materials sandwiched between itself and the cylinder as they pass through the transfer zone. The pressure is exerted along the narrow transverse segment of the receiving sheet and the photoelectrostatic member.

The operable pressure range in which successive transfers may be made without disturbing the image receiving medium has as an upper pressure limit 8 pounds per square inch pressure calibrated as an empirical pressure value for a roller of given resiliency, and a lower pressure limit that is sufficient to bring all the transferable powder into contact 'with the receiving sheet. Optionally, a field may be imposed across the transfer zone, that is, between the surface of the transfer roller electrode and the conductive backing or support of the photoelectrostatic member, the operable range being from 300 volts to as high as 3000 volts.

The process described herein can operate successfully without any applied field across the transfer station. It has been found that the degree of powder transfer is enhanced, and therefore more dense images are transferred under the influence of a potential applied between the transfer roller surface and support for the photoconductive layer. The suggested ranges will depend on the type of photoconductive medium employed, as discussed in greater detail hereinafter.

The latent image represents one form of an image receiving medium useful in the duplicating process of this invention. The concept of an image receiving medium contemplates the use of a fixed or fused image which may be charged so that it will attract the electroscopic particles. In general, reference to an image medium in this description represents any medium on a photoconductive layer capable of being repeatedly developed to a transferable image after a preceding image has been transferred to a receiving sheet.

It is a general object of this invention to provide a high speed, simplified method for making multiple copies from a single photoelectrostatic member having an imagereceiving medium upon which a transferable material image may be created employing electrostatic imaging and controlled transferring techniques.

It is a further object of this invention to provide a simplified, highly reliable, high speed method for making multiple copies from a latent image bearing member by sequentially developing the latent image and transferring the resulting material image under controlled pressure.

It is a further object of this invention to provide a method of making multiple copies from a single photoelectrostatic member having a fixed material image thereon capable of selectively attracting electroscopic particles forming a transferable image which may be transferred to a receiving sheet under controlled conditions of pressure in the presence of an electrical field.

It is a specific object of this invention to make multiple copies from a single photoelectrostatic member by creating an initial latent electrostatic image from which a transferable image may be produced and from which successive transferable images may be produced by reestablishing the same image and re-developing a transferable image for each copy produced, each transfer being carried out under controlled pressure and in the presence of an electrical field.

It is another object of this invention to provide a method of making multiple copies from a latent image bearing photoelectrostatic member in which development of the transferable image portion is accomplished :by a magnetic brush applicator operating under the influence of an adjustable field between the brush and the charged image portions.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more easily understood by reference to the following detailed description and to the drawings in which:

FIG. 1 is a diagrammatic view illustrating the step of charging of a photoelectrostatic member;

FIG. 2 is a diagrammatic view illustrating the step of exposing a charged photoelectrostatic member;

.FIG. 3 is a diagrammatic view illustrating the step of developing a charged image;

FIG. 4 is a diagrammatic view of a duplicating apparatus embodying this invention;

FIG. 5 is a diagrammatic view of a typical image fusing apparatus;

FIG. 6 is an illustration of another technique of fixing a developed image;

FIG. 7 is a diagrammatic view of a unitary duplicating apparatus embodying all the steps necessary to carry out the process of this invention;

FIG. 8 is a diagrammatic view of a duplicating apparatus capable of carrying out another embodiment of the invention;

FIG. 9 is a perspective view of the pressure setting controls associated with the transfer station;

FIG. 10 is a schematic circuit diagram of a simplified control system for controlling the operation of the duplicating apparatus shown in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is shown a photoelectrostatic member identified generally as 20, being applied a blanket electrostatic charge from a charging assembly identified generally as 22. The step of applying a charge to photoelectrostatic member 20 is the first in the duplicating process. The photoelectrostatic member 20 comprises a photoconductive layer 24 comprised of photoconductor particles, such as zinc oxide, dispersed in an insulating resin binder applied to a base support 25, such as paper or a plastic film.

The use of zinc oxide in a resin binder system is one form of a photoelectrostatic member that could be employed representative of inorganic photoconductive metal ion containing crystalline materials. It should be pointed out that other inorganic materials can be used in a wide range of resin binders well known in the art. In addition to inorganic photoconductive materials it is contemplated that the photoelectrostatic member may be formed of an organic photoconductive polymeric material such as poly- (N-viny'lcarbazole) or copolymers thereof. It has been found that the novel concepts of this invention are not restricted to any specific photoconductive material.

The charging assembly 22 includes oppositely positioned spaced apart corona discharge electrodes 26 and 28 connected to a high voltage supply 30. The upper electrode 26 is connected to the negative side of the source and the lower electrode to the positive terminal with the member 20 passing therebetween with the photoconductive layer 24 facing electrode 26. The application of 4.5 to 6.0 k.v. from the high voltage charge emission source from the electrodes takes place thereby imparting a blanket negative charge to the surface of member 20. The step of charging takes place in the dark in order to avoid premature discharge of the layer 24.

The charged photoelectrostatic member 20 is then exposed, as shown in FIG. 2, under dark room conditions, to a pattern of light and shadow created by illuminating a graphic original 32 having image portions 34. The graphic original 32 is illuminated by a suitable radiant energy source such as a pair of lamps 36, producing a light pattern of the graphic original in which the shadow portion corresponding to the image areas 34 is cast by a lens system 8 onto the charged layer 24.

As is well understood in the art, the light struck portions of the charged photoconductive layer 24 are dissipated leaving charged portions 40 which correspond to the image portions 34 of the graphic original, said charged portions 40 being the image receiving medium.

The exposed member 20 with the image receiving medium 40 is next developed using the magnetic brush-type developing apparatus as shown in FIG. 3. The member 20 is placed against a conductive plate 42 which is connected to ground and brought into contact with a magnetic brush assembly 44.

The developer brush 45 is formed on the periphery of the rotatably mounted applicator roll 46 having disposed within a stationary magnetic means 48. The applicator roll 46 is mounted in a trough-like container 50 in which is provided a mass of developer mix 52 comprising iron particles mixed with heat-fusible or pressure-responsive highly colored electroscopic resin powder. The magnetic means 48 is preferably a permanent magnet equal in length to the transverse dimension of the paper passed through the developer apparatus. It consists of north and south pole pieces positioned relatively close to the inner periphery of the applicator roll 46 producing a field of magnetic flux having zones of concentration at the top and bottom of the roll. As the roll turns it picks up the magnetically attractable iron particles which carry along the resin powder held on the iron by triboelectric forces.

In the zone of greatest magnetic flux the developer mix forms up into a brush 45 on the applicator roll 46 and as it moves out of range the mix falls into the trough 50 to be intermixed with the main body of the mix 52.

It will beappreciated that other developing techniques may be effectively employed to develop a material image such as cascade development and powder cloud.

In conjunction with the magnetic brush developing technique there is provided means for controlling the development of the electrostatic image by imposing an electrical field between the brush and photoconductive layer. Referring to FIG. 3, a DC source 54, such as a battery or other suitable supply, is connected to the metal applicator 46 via the connector 56 and potentiometer 58. The applied brush biasing effect can thus be adjusted to vary the strength of the biasing field.

The applied field tends to bias the electroscopic particles entering the field. By applying an electric field in the same polarity as the charge on the surface of the photoconductive layer 22, in addition to the field emanating from the image, the contrast of the reproduction is greatly enhanced. The brush biasing technique permits correction for under-exposure to light by increasing the biasing voltage to the brush. The increased voltage, through the potentiometer control 58, tends to overcome the residual charges that have the same polarity as the brush.

Thus far in the description the various apparatus and processing steps are well known. The photoelectrostatic member 20 has developed thereon a loosely adhering material image 40' which may now be transferred to a receiving sheet.

Referring to the schematic representation in FIG. 4, the member 20 having thereon the developed powder image 40' is mounted on the conductive rotatable cylinder 59 having associated therewith an electrical field producing transfer assembly 60. The assembly 60 includes a transfer member 61 comprising a soft conformable rubber layer 62, inch to inch thick, and having a resistivity in the range of 10 and 10 ohm-centimeters, bonded to a conductive metal core or shaft 64 about inch in diameter. The rubber layer should have a hardness that falls within a durometer reading range of 20 to 50 units, preferably in the range of 30 to 40 durometer reading. The durometer readings referred to are measured on a Shore A scale as described in ASTM Standards D-676 concerning rubber hardness. The axis of rotation of the drum 60 is parallel to the shaft 64. It has been found that the transfer of the electroscopic particles is enhanced by providing a potential difference between the surface of the transfer roller and the conductive support of the photoconductive layer. The potential difference that can be applied varies somewhat with the type of photoconductive material. It is believed that this may be related to the light exposed resistivity of the layer. The higher the light exposed resistivity, such as in the case of organic photoconductive polymeric materials which might range from 10 to 10 ohm-centimeters, the greater is the tendency to accept a surface charge. The useful voltage range to be applied at the transfer station for organic photoconductors is from 300 to 1500 volts within the operatin range of 0 to 3000 volts.

In the case of zinc oxide in resin binder systems the light exposed resistivity is in the range of 10 to 10 ohmcentimeters, and the useful voltage range at the transfer station would be from 800 to 3000 volts within the operattng range of 0 to 3000 volts.

It will be appreciated that, in order to achieve the particular potential across the elements at the transfer site, the input to the core 64 will have to be in excess of the desired potential to be applied across the transfer zone. It will be understood that the voltage input to the conductive core 64 can vary widely depending on the conductivity of the transfer roller surface, the area of contact between the elements and the electrical properties of the materials sandwiched together at the transfer zone and still come within the operable range of 300 to 3000 volts. It is believed that by specifying the potential as being across the transfer roller surface and the conductlve support of the photoconductive layer there is elimmated from consideration the various elements between and including the core 64, and the transfer roller surface affecting the potential across the transfer zone.

In the circumstance that the powder image is transferred from a zinc oxide resin binder layer on a paper substrate to a paper receiving member and the transfer assembly is similar to the assembly 60 (FIG. 4) described earlier, the applied voltage to the core '64 from the source 66 should be in the range of 1000 to 3500 volts.

In the case of the organic photoconductive medium the photoconductive layer is applied to a conductive base support such as treated paper or metal foil and the layer makes contact with the receiving member, the applied voltage to the core 64 should be in the range of about 500 to 1750 volts in order to produce a potential gradient between the two surfaces in the range of 300 to 1500 volts.

The photoelectrostatic member 20 is clamped around the outside of the cylinder 42 with the unfixed image portions 40' facing away from the center of the cylinder. A receiving sheet 68, which is plain paper such as bond paper, is fed in timed relation to the rotation of the cylinder so that the lead edge of the receiving sheet is in registration with the lead edge of the photoelectrostatic member 20 as the two elements 68 and 20 are brought into intimate contact under pressure between the cylinder 59 and the member 61 in the presence of an electrical field imposed by the field producing assembly 60, connected to the DC source.

The construction and use of the transfer electrode assembly 60 is such that voltages in the range of 0 to 3500 volts can be applied without resulting in a significant current flow in the system so as to produce a charge on the member 20. The soft rubber layer 62 maintains the trans- 7 fer member 61 in close contact with the surface of the photoelectrostatic member 20 which is disposed around the periphery of cylinder 59. This facilitates bringing all portions of the receiving sheet into intimate contact with the photoelectrostatic member. Under this condition of uniform intimate contact between the narrow transverse areas of the receiving sheet 68 and the powder image bearing photoelectrostatic member 20 complete transfer of the powder image 40' takes place in the presence of the field impressed by connecting the core 64 of the transfer member 61 to a DC voltage supply and applying a voltage in excess of 800 volts for inorganic photoconductive systems, preferably in the range of 1000 to 3500 volts, and for organic photoconductive systems a voltage in excess of 300 volts, preferably in the range of 500 to 1500 volts. With transfer rolls of different diameters than referred to herein, or if the thickness of the conductive layer is varied, or the resistivity of the material formed on the core changes, the amount of voltage input will change in order to achieve the potential across the transfer site as defined herein.

It has been found that the pressure between the transfer member and the cylinder when transferring the powder image is a critical factor and must not exceed about 8 pounds per square inch, representing an empirical pressure between the cylinder and transfer member whose measurement and adjustment will be described in greater detail hereinafter. The lower limit must provide suflicient pressure so that the powder contacts the receiving sheet and also to drive the sheet through the transfer zone without smearing the loose image. The preferred range of pressure is from about 3 to about 5 pounds. The ability of the image receiving medium to continue to function in the duplicating mode depends on the pressure not exceeding about 8 pounds per square inch. When the pressure exceeds this limitation, the application of voltage to the transfer site at any level is ineffectual to preserve the image.

Under the proper level of pressure set forth herein the duplicating process produces reproductions which are of the proper image density and free of spurious deposition of toner in the background over repeated cycles. Under the proper conditions duplicating runs well exceeding 1000 copies can be routinely achieved.

As the pressure contact is increased beyond the critical level, the image portion, in the latent image embodiment, is found to rapidly deteriorate, that is, the potential in the image portion rapidly drops off. Concurrent with the dissipation of the charge in the image portions there occurs the creation of a charge build-up in the non-image area which attracts the electroscopic powder. Below the lower limit, which is approximately 2 pounds per square inch pressure, the density of the transferred image is oor.

p In the fixed image mode of making transfers as the pressure goes beyond 8 pounds, a charge build-up is observed in non-image areas or the background areas of the photoconductive surface, which is manifested by the attraction of the powder to these areas. Illumlnation of the photoconductive surface does not appear to dissipate the charge.

The pressure values represents an emplrical pressure arrived at by calibrating the transfer member under the actual conditions at the transfer station. Since the transfer roll is made up of a compressible material, the area of contact between the cylinder and the roll will vary in accordance with the force with which the roll is being pressed against the cylinder.

A roll of a given compressibility may be calibrated 1n terms of adjusting the gap opening between the roll and the cylinder to a predetermined setting 1n order to exert the necessary contact pressure in pounds per square lnch against the sheet elements passing between the rollers.

Given a roll of known durometer reading (Shore A the amount of deflection is measured at the area of contact by placing the roll against a uniform flat surface under increasing loads. The deflection is expressed in terms of the decrease in the size of the radius measured along a line normal to the plane of contact. At no load with the roll supported oil. the surface) the deflection is zero. The weight of the roll itself resting on the surface results in some deflection and by successively adding known increments of weight to the ends of the shaft the force is increased and the amount of observed deflection for each weight level recorded.

In this manner a deflection-pressure calibration may be developed for a particular roll. The deflection observed for a given load may be translated into an empirical pressure value by first calculating the aera of contact between the roll and the flat surface, and then dividing the area by the total applied load. The area of contact is represented by the product of the length of the portion of the roll that contacts the paper by the Width of the flat porion of the roll.

A typical calibration for a rubber covered transfer roll having a Shore A durometer reading of 35 and radius of 0.565 inch making contact with the paper elements along 9 inches of its length is as follows:

Deflection Calibrated in roll Pressure Contact values Width of flat contact pressure AR inches 2[R (RA R) area (in?) (lbs/in!) To achieve the desired contact pressure, the gap between the roll and cylinder is adjusted by means of a precision feeler gauge to a setting which is the difference between the combined paper thickness and the roll deflection that corresponds to the calibrated roll pressure.

Hence, if the combined paper thickness is 0.008 inch and the pressure to be applied is 5.80 pounds per square inch, the corresponding deflection from the calibration chart is .002 inch. The gap between cylinder and transfer roll should then be preset to an opening of .006 inch.

The transfer roll mounting, to be described in greater detail hereinafter, is equipped with calibrated micrometer adjusting means so that the gap opening may be precisely preset relative to a zero position.

In the circumstance that a softer rubber layer is used, the calibration data will yield greater values for deflection (AR) for the same loads applied. Understandably, the pressures, when using a softer rubber, will be less since the area over which the force is being applied is greater. Following the calibration procedure outlined above, adjustment of the transfer roll to produce the desired pressure requires bringing the transfer roll into contact with t e cylinder to produce an initial flat in the absence of the sheet elements. A negative value for the gap opening would mean that the transfer roll is to be adjusted against the cylinder so that in its start condition, i.e., prior to the passage therethrough of a known thickness of paper, there is a known contact area and, hence, an initial pressure.

It should be pointed out that transfer of the image under proper pressure conditions continues absent the applied potential between transfer roll surface and support for the photoconductive layer for duplicating runs exceeding 1000 copies, and when runs of greater and more uniform image density are desired it is recommended that the appropriate potential be applied at the transfer station.

At low pressures, less than about 2 pounds per square inch, the transfer is non-uniform because of non-uniform contact and increasing the voltage is ineffective to correct insufficient pressure.

After the particle image has been transferred to the receiving sheet, it may be fixed thereon using the conventional technique of fusing by heat. As shown in FIG. 5, the receiving sheet 68, having the unfused image portions 40 thereon, is passed beneath a bank of radiant heaters 70 which raises the temperature of the electroscopic resin powder to its fusing temperature whereupon the resin softens and adheres, being permanently bonded to the paper after it cools.

Another technique for fixing the powder image is shown in FIG. 6 in which the receiving sheet 68 with a powder image 40' is passed between a pair of nipping polished steel rollers 72 in pressure contact. The powder image is fixed to the paper under high contact pressures exerted by the rollers. Optional use may be made of a pressure responsive electroscopic resin powder such as disclosed in copending application, Ser. No. 596,476, filed Nov. 23, 1966, in the name of Loren E. Shelffo and assigned to the same assignee as this invention. As the receiving sheet emerges from between the pressure rollers, the image is permanently fixed to the surface of the receiving sheet 68.

Referring to FIG. 7 of the drawings, there is shown a complete apparatus which incorporates all the processing steps necessary to make reproductions on plain paper from an image bearing photoelectrostatic member. The photoelectrostatic member 20 is clamped on the outside surface of the rotatable cylinder 80. The cylinder 80 has disposed adjacent its periphery and its rotational path various stations, each equipped with the necessary instrumentalities to process the photoelectrostatic member 20 as it passes each station.

The photoelectrostatic member is clamped on the cylinder by conventional clamping means 81. It will be understood that in practice the member 20 will wrap around most of the outer circumference of the cylinder, however, for purposes of better illustrating the process, the diameter of the cylinder has been exaggerated to show a copy sheet at each station. Commencement of rotation causes energization of the charging station 82 which lays down a blanket electrostatic charge on the surface of the member. This renders the surface light sensitive and hence, it is imperative that the member be processed under darkroom conditions.

At the charging station 82 there is provided a series of fine wire electrodes 84 stretched Within a conductive shield 86 connected to ground. The corona wires are connected to a DC high voltage source capable of adjustment in the range of 3000 to 6000 volts, laying down a sensitizing charge on the photoconductive layer of the member 20 in the range of 400 to 600 volts as it passes beneath the emitting wires while the back surface is in contact with the conductive cylinder which serves as the other electrode to ground 88.

The charged member 20 proceeds next to the illuminating station 90 where a pattern of light and shadow generated by illuminating an original subject 92 having image portions 93 using a pair of lamps 94 is projected thereon through a lens system 96. The cylinder 80 is momentarily stopped in its rotational path to receive the pattern. Thus, there is produced an image receiving medium 95 in the forme of a latent image. In order to avoid distortion of the image, the original 92 is mounted for illumination in a curved position which corresponds to the degree of curvature of the cylinder.

At the conclusion of the illumination step the cylinder resumes its rotative motion conveying the latent image bearing photoelectrostatic image to the developing station 97 where the light exposed member, which now bears a latent electrostatic image, is developed into a transferable material image 95'. The developing mechanism is a conventional magnetic brush 98. The magnetic brush unit is well known in this art.

It has been found desirable to impose a biasing voltage on the magnetic brush proper, as has been described earlier. A variable DC source 99 applying a potential between the brush and the image receiving medium in the range of 50 to 1200 volts, with one side of the brush connected to ground, enhances the quality of the image. Deficiencies in exposure can be compensated for by either increasing the voltage when an underexposed condition exists and decreasing, or even reversing the voltage signs, when the member 20 is over-exposed.

A transfer station 100 is provided beyond the developing station and convenient to the supply of receiving sheets 102. The receiving sheets 102 are stacked in a feed tray 104 equipped with rubber feed Wheels 106 which are operated to advance the uppermost sheet in the stack along a path tangential to the cylinder feeding the sheet to the transfer station 100. The operation of the feed wheels 106 is in timed relation to the rotation of the cylinder so that the lead edge of the copy sheet 102 is in registration with the lead edge of the member 20 as they arrive at the transfer station.

At the transfer station there is provided a transfer member 108 having a soft rubber outer layer 110 and a conductive core or shaft 112 connectedto a DC power supply 114. The rubber outer layer is in pressure contact with the periphery of the cylinder. The construction and operation of the transfer member has been described previously in connection with FIG. 4. As the receiving sheet passes between the cylinder and the transfer member, the powder image is transferred to the receiving sheet The final step involves fixing the transferred image on the receiving sheet by means of pressure rollers or using any of the aforedescribed fixing techniques.

Up to this point the cylinder 80 has made one revolution in which the member 20 has had created thereon an image receiving medium from which a single transfer was made onto plain paper. The conditions at the transfer station require that optimum pressure be maintained between the transfer member 108 and cylinder 80 in order to effect a satisfactory transfer of the powder image 95' onto the receiving sheet 102 without disrupting the latent electrostatic image. For duplicating runs of uniform and more dense images it is desirable to impose a potential between the surface of member 108 and the conductive backing 25 of the photoelectrostatic member 20 in the range of 800 to 3000 volts for resin binder systems and 300 to 1500 volts for organic photoconductive polymeric systems.

Further rotation of the cylinder 80 launches the duplicating cycle in which the image receiving medium requires that the developing station 97 and the transfer member participate in conjunction with feeding of the receiving sheets 102 from the supply tray 104. Under darkroom conditions, whereby the image receiving medium 95 is protected from light exposure, it is redusted with the electroscopic powder by the magnetic brush 98 and transferred to a receiving sheet fed in timed relation.

It will be appreciated that the first cycle of producing the image receiving medium is slower by comparison to the duplicating cycle. The photoelectrostatic member can be imaged in a range of from 5 to 12 seconds. Thereafter, the second and subsequent transfer reproductions can be carried out at the rate of about 6000 to 7000 copies per hour.

Another embodiment of this invention permits the duplication of a graphic original on plain paper by making transfers from an image receiving medium comprised of a fused or fixed image on the photoelectrostatic member. The attribute of working with a fixed image is that the photoelectrostatic member is no longer sensitive to light and may be worked in ordinary room light.

Referring to FIG. 8, the apparatus shown is adapted to operate using a fixed image receiving medium in which the image is heat-fused after development during the first cycle by directing a source of heat 110, such as a 500- watt T3 infrared lamp manufactured by General Electric Company. Such a heat source is suflicient to fuse known powders, fusible over the range of 70 C. to 250 C. The heat lamp extends transverse the extent of the cylinder 120 and coextensive the width of the photoelectrostatic member 20.

The photoelectrostatic member 20 is mounted on the cylinder 120, charged in the dark at a charging station 122, exposed to a pattern of light and shadow through a lens 121 by the illumination with a pair of light sources 124 of the graphic original 125 having image portions 126. This produces a latent electrostatic image 127 corresponding to the image portions 126. The charged image portions 127 are next developed by the application of thermoplastic electroscopic resin particles via the magnetic brush 128 producing a transferable material image 127'. The material image is fused onto the member 20 by the radiant heaters 110 which completes the creation of the image receiving medium 127 of the duplicating cycle. Up to this point the process must be carried out under darkroom conditions.

The next rotation of the cylinder 120, carrying the fused image bearing member 20, commences the duplicating cycle. The duplicating cycle is initiated by activating the charging unit 122 where a blanket electrostatic charge is laid down on the surface of the member 20 charging both the fused image areas 127' and background or non-image areas 129.

Further rotation of the cylinder brings the charged member 20 to the exposure station 123 where during the duplicating cycle the sheet is given a flood exposure (patternless) by the light sources 132 directed onto the cylinder. The flash exposure serves to discharge the background or non-image areas 129 leaving the fused image portions 127' carrying an electrostatic charge. The fused thermoplastic resin binder readily accepts a charge because of its high resistivity which is in the range of ohm-centimeters to 10 ohm-centimeters. Since the thermoplastic resin is not photoconductive, it does not respond to the flood exposure as do the background areas, but retains an electrostatic charge.

The charged fused image portion has applied thereto an electroscopic thermoplastic powder which selectively adheres to the fused image 127'. It will be appreciated that the image 127 now has superimposed thereon a loose transferable powder image 131. As the redusted, redeveloped number 20 approaches the transfer station 133, a receiving sheet of plain paper 134 is synchronously fed from the supply tray 136 in timed relation so that the two sheets are in registration as they proceed between the transfer member 138 and cylinder 120.

The outer layer of the transfer member is similar in construction to the transfer member described earlier in connection with the latent image technique of duplicating from a photoelectrostatic member. The transfer member 138 comprises a conductive core 140 having an outer wrap or layer 142 of a soft compressible rubber, similar in construction to the transfer member described in connection with FIG. 4. The core 140 is connected to a high DC voltage source 143. As the set comprising a sheet of plain paper 134 and the imaged member 20 carrying a redeveloped image, pass between the member 138 and the cylinder 120, the redeveloped fused image is brought into pressure contact against the receiving sheet 134. Under the proper conditions of pressure contact, such that all portions of the master are brought into contact with the receiving sheet, but insufficient to deform either of the sheet elements, a satisfactory transfer of the electroscopic powder is effected. To achieve larger duplicating runs where the transferred images are of a consistent density, it is desirable to apply a potential between the surface of the transfer roll 138 and the conductive backing 25 of the photoelectrostatic member .20, preferably in the range of 300 to 3000 volts.

Thereafter, the transferred image on the receiving sheet is fixed by heat fusing or pressure fusing by process- 12 ing through the construction shown in FIG. 5 or 6, respectively.

In another embodiment the apparatus described in connection with FIG. 8 may be adapted to a multiple copying apparatus in which the image receiving medium on the photoelectrostatic member is created by re-exposure after each transfer to a pattern of light and shadow of the same original. Referring to FIG. 8, the photoelectrostatic member 20 is clamped on the cylinder and, as it passes beneath the charging unit 122, it receives a blanket electrostatic charge.

The charged member 20, which is now sensitive to light, is exposed to a pattern of light and shadow created at the illumination station 123 by illuminating the original subject 125 with the lamps 124 and projecting the image through a lens 121 onto the charged member under darkroom conditions. This is accomplished by a flash exposure of the original without stopping the rotation of the cylinder.

The photoelectrostatic member, bearing the latent electrostatic image, is next developed at developing station 128 in the same manner as described in connection with the previous embodiments.

In this mode the fusing unit 110 is not employed. A transfer station 133 is provided which includes the transfer member 138 having a soft rubber outer layer 14-2 and a conductive core 140 connected to a DC power supply 143 at which transfer of the powder image occurs in accordance with transfer principles and conditions as set forth hereinabove. As the final step in the process the transferred image is fixed on the receiving sheet by pas sage through a pair of highly polished steel rolls 72, applying contact pressure in excess of 20 pounds per square inch.

All of the steps are repeated in carrying out the next reproduction including charging the flash exposure of the same original, development, transfer and fixing; all carried out in the dark. The process may be continued for long runs. The preferred photoconductor in this instance is Zinc oxide because of its rapid recovery in the dark after light exposure to the insulating level where it can again accept a charge in the range of 300 to 600 volts.

Referring to FIG. 9, there is shown the pressure adjusting control whereby the transfer member 108 may be adjusted relative to the cylinder 80 in order to apply a controlled pressure within the critical limits called for in the instant invention.

For purposes of brevity only one such control will be described, it being understood that two such controls are provided, one for each side of the transfer station.

While the description will refer to FIG. 7, it will be understood that the same pressure adjusting control may be employed for all the embodiments described hereinabove.

The cylinder 80 and the transfer member 108 are mounted on shafts 172 and 174, respectively, which are rotatably mounted between the side plates 176 (one shown). The shaft 172 is rotatably received in a bearing 178 in the side plate 176.

The free end of the transfer member shaft 174 extends through an elongated opening 173 in the side plate 176, being rotatably received in an arm 180 juxtaposed the opening 173. The arm 180 is fastened to the plate 176 by means of a threaded fastener 18 2 being maintained in spaced relation from the plate by means of a spacer bushing 184, thereby permitting the arm to pivot about the fastener 182. The arm 180 is provided with an extension 186 having attached thereto one end of a coiled spring 190, the other end of said spring being fixed to a bracket 192, secured to the side plate 176, tending to urge the arm 180 to pivot in an upward direction.

The coiled spring 190 is secured to the bracket by means of a threaded shaft portion 194 and adjusting nut 13 196 which provides an adjustable tension control for the spring.

In the space between the side plate 176 and the armextension 186 there is provided a gap adjusting means in the form of a micrometer 198, having an adjustable measuring head 200 adjusted by turning a barrel portion 202 and a fine adjustment head 204. The micrometer adjusting means is supported on a bracket member 206 which locates the micrometer in a plane parallel to the plane of movement of the arm 180. The measuring head 200 of the micrometer is arranged to engage a metal stop 208 projecting from inside the extension arm 186. .It will be appreciated that the pivoting action of the arm 180 swinging upward is impeded by the stop 208 engaging the measuring head 200 so that the stop 208 serves as an anvil relative to the measuring head 200.

To best illustrate the operation of the pressure setting control 170, reference will be had to the earlier example where the calibrated contact pressure to be applied between the transfer member 108 and the cylinder 80 is 5.80 pounds per square inch.

Referring to the calibration chart described earlier, it is determined that a pressure of 5.80 pounds per square inch calls for a roll deflection of 0.002 inch. At a combined paper thickness of 0.008 inch the gap between the transfer member and the cylinder is to be 0.006 inch.

To achieve this setting, the adjusting barrel 202 of the micrometer 198 (one at each end of the transfer station) is adjusted to provide a sufficiently large gap opening. Through the use of precision feeler gauges, having a known thickness, the gap between the member 108 and the cylinder '80 is closed by turning the adjusting barrel 202 permitting the arm 180 to move upward until the shim stock just begins to be frictionally held between the cylinder 80 and the transfer member 108. Assuming, for the purposes of this example, that the thickness of the shim stock is 0.005 inch, then the gap opening between the cylinder and the transfer member in this first adjustment will correspond to the shim thickness.

The reading on the micrometer barrel 202 at this first adjusted setting is observed and becomes in practice a zero reading. For purposes of illustration we will assume that the readings on the micrometers at each side of the transfer station at this first adjustment are, respectively, 0.0028 inch and 0.0044 inch. Since the gap is to be adjusted to a setting of 0.006 inch, a second setting in which the gap is opened an additional 0.001 inch (the difference between 0.006 inch and 0.005 inch) is required. Accordingly, the barrel settings are adjusted to a new reading of 0.0018 inch and 0.0034 inch, respectively, which gives a final gap opening of 0.006 inch.

Passage through this opening of the photoconductive member and the receiving sheet, having a combined thickness of 0.008 inch, will, according to the calibration chart exert a contact pressure of 5.80 pounds per square inch.

Referring now more specifically to FIG. of the drawings, there is illustrated a control circuit which is indicated generally as 220 and which is adapted to control the operation of the duplicating apparatus shown in FIG. 8 of the drawings. In general, the control circuit 220 converts the mode of operation of the duplicating apparatus between a first mode in which a powder is fixed on the photoelectrostatic or photoconductive member 20 on the cylinder 120 during its first cycle of rotation and a second mode in which a powder image is developed on the fixed image and transferred to one of the sheets 134 during each of the succeeding cycles of rotation of the cylinder 120 until the desired number of copies has been produced.

The control circuit 220 is shown in simplified form and includes, in addition to the illustrated components, suitable means for controlling the energization of various drive motors-and potential sources, such as the biasing potential sources for the corona charging assembly or station 122 and the magnetic brush 128. The circuit 220 also includes a motor control circuit 222 for driving the drum or cylinder 120 at a slow rate or intermittently during the production of the image on the member 20 or continuously at a higher speed when powder images for transfer to the sheets 134 are produced. To synchronize the various operations of the components spaced along the path of rotation of the cylinder 120, a plurality of ca ms 224, 226, 228, and 230 are rotated in synchronism with the cylinder 120 to selectively actuate or close four switches 232, 234, 236, and 238, respectively, at different intervalswithin each cycle of rotation of the cylinder 120. The control circu1t 220 also includes a copy counter 240 of conventional construct on that is manually adjusted to the number of copies desired of the graphic original. When a copy number or order 1s fed into the copy counter 240, a switch 242 1s closed to provide a source of holding potential for certain components of the control circuit 220. The contacts 242 are opened when the requested number of copies has been produced by the duplicating apparatus. I

When the duplicating apparatus is to be placed in operation and after a time delay suflicient to Insure the operability of all of the components of the duplicating apparatus, which delay can be provided by conventional time delay means, a momentary start swltch 244 is closed to energize and operate a start relay 246. The operation of the start relay closes two pairs of contacts 248 and 250. The closure of the contacts 250 completes a holding circuit for the start relay 246 extending to negative potential at the closed contacts 242. The closure of the contacts 248 supplies potential to the cam controlled sw1tches 232, 234, 236, and 238 and also extends a start signal or potential to the motor control circuit 222. The oncuit to the motor control circuit 222 includes a pair of normally closed contacts 261 on a transfer relay 260.

The energization of the motor control circuit 222 starts rotation of the cylinder 120 so that the member 20 1S moved past the charging assembly 122 to permit one surface thereof to be uniformly charged. The motor control circuit 222 then moves the member 20 to the exposure station and, when this sheet reaches the exposure stat on, the cam 224 closes the contacts 232 so that a circuit 1s completed through the closed contacts 232 and a pair of normally closed contacts 263 on the transfer relay 260 to a control circuit 270. The circuit 270 controls the illum nation of the lamps 125 to produce a latent electrostat c image on the member 20 corresponding to the graphic original to be recorded. The period during which the lamps 124 are energized can be controlled by timers 1n the circuit or the dwell of the cam 224.

Afterthe member 20 is exposed, continuing rotation of the cylinder 120 moves the member 20 to the developing station at which the latent electrostaticumage is developed by the application of toner or particulate developer material by the magnetic brush 128. Continuing rotation of the cylinder 120 advances the image bearing surface to the fusing or fixing station containing the source of heat 110.

As the member 20 reaches this station, the contacts 234 are closed by the cam 226 and complete an energizing circuit including a pair of normally closed contacts 265 on the transfer relay 260 for energizing the heat source in dependence on the nature and quantity of heat required. The motor control circuit 222 can provide a dwell in this position or the cylinder can be moved slowly during this portion of its path of rotation to fix or place the powder image in a permanent form. The motor control circuit 222 then rotates the cylinder 120 to advance the member 20 through the transfer station and back to the point at which the member 20 containing the fixed image s again provided with' a uniform electrostati charge. During this rotation, the cam 230 momentarily closes the switch 236, but this switch closure is without function at this time.

When the cylinder 120 reaches its normal position at the completion of a single cycle of revolution, the cam 230 momentarily closes a pair of contacts 238 to prepare the control circuit 220 for operating the duplicating apparatus in its second mode. More specifically, when the contacts 238 are momentarily closed by the cam 230, an operating circuit for the transfer relay 260 is completed over a circuit including a pair of normally closed contacts 268. The contacts 268 and a pair of normally open contacts 267 provide a make-before-break contact arrangement. Thus, when the relay 260 is operated to open the contacts 261, 263, 265, and 268 and to close a plurality of normally open contacts 262, 264, 266, 269, and 269A, as well as the contacts 267, the closure of the contacts 267 completes a holding circuit for the relay 260 extending to the closed contacts 242 prior to the opening of the contacts 268 to interrupt the above described operating circuit. This circuit holds the transfer relay 260 operated until the contacts 242 are opened by the copy counter 240.

The opening of the contacts 263 disables the circuit for illuminating the lamps 125, and the opening of the contacts 265 disables the circuit for energizing the heat source 110. The closure of the contacts 269A prepares a path for feeding counting signals to the copy counter 240.

The closure of the contacts 269 supplies an operating signal to the potential supply 114 for the transfer roller. The receipt of the negative potential from the closed contacts 242 controls the power supply 114 to supply a biasing potential on the order of 1000 volts to the conductive core 140 of the transfer roller. Thus, the energization of this transfer roller assembly can be deferred until such time as the duplicating apparatus is operated in its second mode in which powder images are transferred from the member to the copy sheets 134.

The opening of the contacts 261 removes the first mode control signal from the motor control circuit 222, and the closure of the contacts 262 supplies a control signal to the circuit 222 indicating that copy production from the fixed image is to be initiated. When the motor control circuit 222 receives the start signal from the closed contacts 262, continuous and high speed rotation of the cylinder 120 is initiated.

During this movement the member 20 containing the fixed image is uniformly charged by the assembly 122 and is moved to the exposure station so that the cam 222 again momentarily closes the contacts 232. The closure of the contacts 232 completes a circuit over the closed contacts 264 for energizing a control circuit 272. The control circuit 272 momentarily energizes the lamps 132. As set forth above, the flash exposure resulting from the momentary energization of the lamps 132 discharges the member 20 except in those areas containing the dielectric image afforded by the fused particulate developing material. These areas retain their electrostatic charge to provide a latent image.

During continuing rotation of the cylinder 120, the cam 226 momentarily closes the contacts 234. However, the prior opening of the contacts 265 prevents the energization of the heat source 110 as the member 20 passes the fusing station.

As the member 20 approaches the transfer station, the cam 228 momentarily closes the contacts 236 to complete a circuit over the closed contacts 266 to a control circuit 276 for the sheet feeding assembly 135. This controls the sheet feeding assembly 135 to feed a paper sheet or web 134 to reach the transfer station in synchronism with the member 20 carried on the cylinder 120. Since the potential source 114 is now energized and a sheet 134 is fed in compression between the roller 138 and the cylinder 120, the powder image carried on the member 20 is transferred to the copy sheet 134, and this image is subsequently fixed on the Sheet 134 in the manner described above.

Continuing rotation of the cylinder moves the member 20 back to its initial or normal position, at which time the cam 230 momentarily closes the contacts 238. The pulse developed by the momentary closure of the contacts 238 is forwarded through the closed contacts 269A to provide an input signal to the copy counter 240 indicating the production of a single copy. This advances the counting mechanism in the counter 240 to indicate the production of the copy.

This operation continues until such time as all of the copies requested by the order and preset in the copy counter 240 have been completed. When the last copy has been produced, the pulse provided by the momentary closure of the contacts 238 advances the copy counter 240 to its final setting, and the contacts 242 are opened. The opening of the contacts 242 releases the start relay 246 and the transfer relay 260 to restore the duplicating apparatus and the control circuit 220 therefor to their normal conditions. The release of the relay 260 coupled with the opening of the contacts 242 also terminates the energization of the potential source 114 for applying bias at the transfer station. The duplicating apparatus can now be used to produce another multiple copy run from a graphic original in the manner described above.

Duplicating equipment constructed in accordance with the processing principles as disclosed herein are eminently successful in turning out consistently high quality reproductions at the rate of 6,000 to 10,000 copies per hour. Because of its simplicity it can be carried out without any special operator training and the economics are such that they can be used in small office installations.

The invention has been described with reference to the specific embodiments thereof for the purpose of illustrating the best modes of operation, but it will be appreciated that modifications may be made in the processing details and the form of the devices without departing from the spirit of the invention.

What is claimed is:

1. The method of making multiple copies of a graphic original from a photoelectrostatic member having a photoconductive surface on a conductive support comprising the steps of:

(a) forming a transferable material image receiving medium by (1) electrostatically charging the photoconductive surface in the dark, and

(2) exposing said charged surface to a pattern of light and shadow corresponding to said graphic original producing a differentially charged surface,

(3) applying electrostatically attractable powder to said surface which selectively adheres to said image receiving medium;

(b) establishing a transfer zone including a transfer member having a resistivity in the range of 10 to 10 ohm-centimeters for transferring said material image to an untreated receiving sheet in which said photoconductive surface and said receiving sheet are brought into contact along a narrow transverse segment while imposing a contact pressure having a range not exceeding 8 pounds per square inch in the area of contact and a minimum pressure suflicient to bring the powder image fully into contact with said receiving element in the area of contact and enhancing the transfer of said electrostatically attractable powder by providing a potential difference between the surface of the transfer roller and the conductive support of said photoconducti-ve member;

(0) feeding a receiving sheet through said transfer zone in timed relation with said photoelectrostatic member transferring the material image to the receiving sheet;

((1) separating said receiving sheet from said photo electrostatic member without disturbing said image receiving medium;

1 7 (e) fixing the transferred image on said receiving sheet; (f) repeating the steps (a)(3) through (e) at least once whereby multiple copies of said original are produced.

2. The method described in claim 1, wherein the potential difference is in the range of about 300 to about 3000 volts.

3. The method described in claim 1, wherein the step of applying said electrostatically attractable powder is by means of a magnetic brush while imposing an electrical potential between the brush and the material image receiving medium in the range of 50 to 1200 volts having a polarity of the same sign as the electrostatic charge on the photoconductive surface.

4. The method described in claim 3, wherein the potential between the magnetic brush and the material image receiving medium is 500 volts.

5. The method described in claim 4, wherein said electrostatically attractable powder is pressure responsive and said transferred image is fixed by passing the receiving sheet through a pair of pressure rollers.

6. The method described in claim 5, wherein the pressure applied at the transfer zone is a first pressure and then applying a second pressure to the receiving sheet bearing the loosely adhered powder image to fix the powder image to the receiving sheet, said first pressure being ineffective against said powder image.

7. The method described in claim 6, wherein the second pressure is in excess of 20 pounds per lineal contact inch.

8. The method described in claim 2, wherein the potential difference is of the same polarity as the charge applied to the photoconductive surface.

9. The method of making multiple copies of a graphic original from a photoelectrostatic copy sheet having a photoconductive surface on a conductive support comprising the steps of:

(a) forming a charge pattern by (1) electrostatically charging the photoconductive surface in the dark, and.

(2) exposing said charged surface to a pattern of light and shadow corresponding to said graphic original producing a differentially charged surface;

(b) forming a transferable material image by applying electrostatically attractable powder to said surface which selectively adheres to said charge pattern;

(c) establishing a transfer zone including a transfer member having a resistivity in the range of 10 to 10 ohm-centimeters for transferring said material image to an untreated receiving sheet in which said photoconductive surface and said reeciving sheet are brought into contact along a narrow transverse segment while applying contact pressure having a range not exceeding 8 pounds per square inch in the area of contact and a minimum pressure sufficient to bring the powder fully into contact with said receiving sheet in the area of contact and enhancing the transfer of said electrostatically attractable powder by providing a potential difference between the surface of the transfer roller and the conductive support of said photoconductive member in the range of about 300 volts to about 3000 volts;

((1) feeding a receiving sheet through said transfer zone in timed relation with said photoelectrostatic member so that the receiving sheet is in contiguous relation withsaid image bearing surface along a narrow transrverse segment transferring the material image to the receiving sheet;

(e) separating said receiving sheet from said photoelectrostatic member leaving undisturbed the charged image portions;

(f) fixing the transferred image on said receiving sheet;

(g) repeating the steps (b) through (f) at least once whereby multiple copies of said original are produced.

References Cited UNITED STATES PATENTS 2,573,881 11/1951 Walkup et a1 101426 2,624,652 1/1953 Carlson 34674 2,843,499 7/1958 Andrus 11717.5 2,844,123 7/1958 Hayford 118-637 3,198,648 8/1965 Trimbur 11717.5 3,267,840 8/1966 Tsutomu Honma et al. 10l-1 3,275,436 9/1966 Mayer 961 3,271,146 9/1966 Robinson 961.4 3,368,894 2/1963 Matkan 96-1 GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner U.S. Cl. X.R. 

